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CT Colonography

Authors: Dr Helen Moore* Prof Parm Naidoo *

What are the generally accepted indications for a CT colonography?

Screening for bowel cancer in people who are considered to have no increased risk of developing bowel cancer.

Failed or incomplete conventional colonoscopy.

When colonoscopy is relatively contraindicated because of significant medical conditions.

Note: Individuals at higher risk of colon cancer because of positive faecal occult blood test, prior colonic neoplasia, family history or those with symptoms or signs requiring exclusion of carcinoma of the colon are better evaluated with optical colonoscopy, particularly because it is likely they will require biopsy/polypectomy. Additionally, patients with known inflammatory bowel disease or a high risk genetic condition, such as hereditary nonpolyposis colorectal cancer (Lynch syndrome), require assessment by a gastroenterologist, as it may be more appropriate to carry out conventional colonoscopy because of the high likelihood that biopsy will be required.

What are the prerequisites for having a CT colonography done?

Note: CTC is not a preferred test in younger people, particularly less than 40 years of age, because of the radiation used in the procedure.

No specific laboratory tests or previous imaging tests are required.

What are the absolute contraindications for a CT colonography?

Acute abdomen or acute diverticulitis.

Bowel obstruction.

Toxic megacolon.

Recent colonoscopy with biopsy or polypectomy – there is increased risk of perforation or exacerbating a subclinical perforation. Discuss with the radiologist, but generally there is a need to wait between 2–6 weeks, depending on the type of endoscopic intervention.

What are the relative contraindications for a CT colonography?

Pregnancy. Only consider CTC if there is a real concern for colorectal cancer and a real risk of perforation with colonoscopy.

Physical weight limitations. These will depend on the capacity of the CT scanning table, which is scanner dependent. Obese patients should check with the radiology practice about table weight limits when they make their appointment.

Patients with inflammatory bowel disease – or a genetic condition, such as polyposis syndromes, that places them at high risk for colorectal cancer; these patients should be first assessed by a gastroenterologist, as they may be best suited for colonoscopy because of the high likelihood of requiring biopsies or endoscopic intervention, such as polypectomy.

Young patients – although the radiation-related risk with CTC is extremely low, patients aged less than 40 have higher susceptibility to radiation effects than older people1.

What are the adverse effects of a CT colonography?

Immediate: Abdominal discomfort, cramping, usually passes quickly. If Buscopan bowel muscle relaxant has been given, there may be the usual side-effects from this, such as transient visual blurring or, rarely, precipitation of glaucoma.

Delayed: Radiation-related cancer induction risk. This is not usually a valid consideration, as the benefit of the test in both screening and symptomatic groups is shown to outweigh this risk. For example, a standard CTC at age 50 years has been calculated to impart a 0.06% lifetime risk of radiation-related cancer; compared with an approximately 30% background chance of cancer – which includes the risk of a colorectal cancer that the CTC is intended to detect and prevent2.

Is there any specific post procedural care required following a CT colonography?

There is no specific post procedural care following a CT colonography

Are there alternative imaging tests, interventions or surgical procedures to a CT colonography?

Other tests to examine the colon are barium enema, and optical (conventional) colonoscopy, flexible sigmoidoscopy and varieties of faecal occult blood tests.

Population-based screening for adenomatous polyps and bowel cancer is a complex and evolving area. Decisions for screening need to take into account cost, diagnostic accuracy and safety.

Brief points regarding colonoscopy:

Compared with optical colonoscopy, there are some advantages and disadvantages to CTC. These include:

The smallest polyps (less than 5mm in size) are not as well seen with CTC as colonoscopy, even though these can rarely be precancerous, or early cancers. The management of very small polyps is controversial and the small risk of perforation and bleeding during a colonoscopy needs to be balanced with the extremely low risk of these tiny polyps being cancerous or precancerous. ‘Even if

CTC correctly identifies a polyp of this size, subsequent endoscopy has a nearly 30% chance of missing it, … and the large majority of these lesions are not adenomas…. The Erlangen Cancer Registry of colorectal polyps and cancers had zero cases of invasive cancer and polyps in over 5000 polyps less than 5 mm in size3,4…’.

Very flat polyps that do not stand out from the bowel wall are harder to see with CTC than with a colonoscopy, especially when they are in the upper part of the colon. These can be very early, curable cancers. They are relatively uncommon (variably reported at 2–10% of all polyps).

Abnormalities in organs outside of the bowel are seen with CTC, but not with optical colonoscopy. This can lead to the need for further investigation of these incidental and often clinically unimportant findings.

CTC is not used for screening after an abnormal faecal occult blood test. Optical colonoscopy is the preferred test because of the higher rate of false positive diagnosis with CTC, and the higher likelihood of the need for polypectomy and/or biopsy in patients who have a positive FOBT result.

Brief points regarding barium enema:

Barium enema is relatively cheap and available, but has much lower accuracy for polyps and cancer, especially when small, compared with optical colonoscopy and CTC. This is no longer regarded as a first-line investigation for suspected bowel cancer in high- or low-risk populations. A contrast enema is useful in delineating the level and cause of an acute large bowel obstruction, especially when colonoscopy is difficult as a result of complete obstruction; for example, sigmoid volvulus or obstructing cancer.

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